I've been 3D printing as a hobby for a couple of years now, and have always heard how 3D printing is great for prototyping, but once you get beyond 10 or 20 pieces it's just not cost competitive with other technologies. I just accepted this to be truth, and have even told some of the customers I've gotten through 3DHubs and other marketplaces the same thing.
Now I'm creating an item for resale and am 3D printing the case. As I'll need 40 - 50 cases at a time I thought I would shop around for other options. But I just received some quotes back on injection molding. The mold price (for half the case) was $15k, plus $10 per unit at a quantity of 50 and $4 per unit at a quantity of 500. Even in the case of the latter, the cost of tooling plus production is $34/piece. 3D printing the same part myself costs $7 in filament, and paying someone else to 3D print it cost $28.
Keeping in mind that 3D printing allows me to make changes to the design on the fly and print the exact number I need without having to worry about volume discounts, I now wonder if I've just been repeating a common misconception. Is 3D printing really noncompetitive for medium (10-500 pieces) production runs? If so, what makes it noncompetitive, given the extremely high up-front cost of IM?
Some small start ups (mainly 3d printing companies) do in fact use 3D printed parts for the production printers they sell. So, at least in some instances, 3D printed parts may be satisfactory for production products.
That being said, other manufacturing methods (such as injection molding) currently have several advantages over 3D printing.
Pros: It can produce parts of different sizes ranging from small, to rather large (like a plastic chair). It provides superior inter-material bonding, takes only a few seconds to produce a single part, and can huge quantities of parts (possibly millions) in a single week. Typically injection molding is often entirely automated, with few moving parts. Small details can be repeated well and parts can be made with many different polymers.
Cons: Some geometries (like a hollow sphere) which can easily be produced with 3D printing, are impossible to produce with injection molds. Start up costs very be high. A typical machine can cost hundreds of thousands to millions of dollars. Mold cavities are costly to produce and require injection molding artifacts to be designed into the part (like ejector pin notches). Also, the mold produces edge lines on the part where the two cavities meet. Defect trouble shooting can be very complex and the ability to produce satisfactory parts can depend on heat flow (which can be affected by things like the weather). This can require climate control to eliminate to reduce waste in scrap parts due to warping and other malfunctions. Sometimes advanced statistics are required to measure dimensions and detect developing problems too complex to deductively trouble shoot because they stem from a fluid thermodynamic issues.
Once 3D printing has time to develop further (as injection molding has), we may see it used more and more for production parts.